Looks like a word question - both end-up the same in Dual Channel MOBOs = no Dual Channel. The only ways to get 6GB is 3x2GB Tri-Channel or 4GB + 2GB, but you stated Dual Channel. Now with DDR3 the 'advantages' of Dual Channel vs non-Dual Channel is pretty slim. The 6GB 2x3GB is meant or X58/LGA 1366 MOBOs, but 'can' work on most Dual Channel MOBOs. Further, mixing 4GB + 2GB the Frequency, CAS Timings and Voltage MUST match-up and still won't run in Dual Channel mode.
In Dual Channel to run, both Channels MUST equal each other: 2GB + 2GB or 4GB + 4GB or 2GB + 4GB + 2GB + 4GB; Channel 0 = Channel 1.
Given the choices 'today' IMO only get 4GB/stick density and in MATCHED SETS.